{-| Module : Acme.Omitted Description : A universal definition of omitted content Copyright : (c) 2013 Joachim Fasting License : BSD3 Maintainer : joachim.fasting@gmail.com Stability : stable Portability : portable A universal definition of \"omitted content\", an alternative to 'Prelude.undefined', and methods for observing whether a definition is merely \"omitted\" or truly \"undefined\". -} module Acme.Omitted ( -- * Usage -- -- $usage -- * A universal definition of \"omitted content\" -- -- $omitted omitted , (...) -- * \"undefined\" redefined -- -- $undefined , undefined -- * Observing the difference between \"omitted\" and \"undefined\" -- -- $observing , isOmitted , isUndefined , isPreludeUndefined ) where import Prelude hiding (undefined) import qualified Control.Exception as E {-$usage This module provides an alternative implementation of \"Prelude.undefined\". To avoid name clashes with the "Prelude", use a qualified import or otherwise resolve the conflict. Use thus @ module AwesomeSauce where import Prelude hiding (undefined) import Acme.Omitted tooLazyToDefine = (...) actuallyUndefinable = undefined main = do merelyOmitted <- 'isOmitted' tooLazyToDefine putStrLn \"Definition was merely omitted\" (...) trulyUndefined <- 'isUndefined' actuallyUndefinable putStrLn \"Definition is truly undefinable\" @ -} {-$omitted The difference between \"omitted\" and \"undefined\" is that the programmer may choose to omit something but she cannot define the undefinable. The former is contingent on the whims of the programmer, the latter a fundamental truth. Operationally, there is no difference between undefined and omitted; attempting to evaluate either is treated as an error. Ideally, programmers would only ever use 'undefined' for things that are truly undefined, e.g., division by zero, and use 'omitted' for definitions that have yet to be written or that are currently not needed. -} -- | Alternative syntax for 'omitted' that has been carefully -- optimised for programmer convenience and visual presentation -- (e.g., for use in printed documents). -- -- Example usage: -- -- > definition = (...) (...) :: a (...) = omitted -- | The universal omitted content operator. -- -- This is sufficient to express _all_ types of omitted content omitted :: a omitted = error "omitted" {-$undefined Lacking a dedicated name for omitted defintions, users of Standard Haskell have been left with no choice but to use \"undefined\" for both the undefinable and the omitted. This makes the standard implementation of \"undefined\" deficient, we cannot be sure what the programmer has intended, only that the definition is missing. Here is an alternate implementation, similar in most every way to the standard implementation, but free from conceptual contamination. -} -- | Denotes all values that are, fundamentally, undefinable. -- -- The implicit (as in not statically enforcable) contract of 'undefined' -- is that it will never be used for merely omitted definitions. -- For that, see 'omitted'. undefined :: a undefined = error "Acme.Omitted.undefined" {-$observing Recent developments in the theory of representing undefined things have made it possible for programmers to more clearly state their intentions, by using our 'undefined' rather than the one from the Haskell 2010 "Prelude". There is, however, still no way to statically ensure that 'undefined' is used correctly. Consequently, 'isUndefined' will return bogus results every now and then (which is why it is modelled as an 'IO' action and not a pure function). Nevertheless, the user can identify incorrect uses of 'undefined' more easily than before. To wit, if @ isUndefined twoPlusTwo = return True @ then, surely, something is amiss. We know that the programmer has made the mistake of believing @2+2@ to be undefined, that she has not simply run out of time or gotten an important phone call while writing down the solution. For backwards-compatibility, we also support detecting the standard implementation of undefined, about which we cannot infer anything except that its evaluation will terminate with no useful result. -} -- | Answer the age-old question \"was this definition omitted?\" -- -- @ -- isOmitted 0 = return False -- isOmitted 'undefined' = return False -- isOmitted 'omitted' = return True -- @ isOmitted :: a -> IO Bool isOmitted = isErrorCall "omitted" -- | ... or is it really 'undefined'? -- -- @ -- isUndefined 0 = return False -- isUndefined 'undefined' = return True -- isUndefined 'omitted' = return False -- @ isUndefined :: a -> IO Bool isUndefined = isErrorCall "Acme.Omitted.undefined" -- | A version of 'isUndefined' for \"Prelude.undefined\". isPreludeUndefined :: a -> IO Bool isPreludeUndefined = isErrorCall "Prelude.undefined" isErrorCall :: String -> a -> IO Bool isErrorCall s x = (E.evaluate x >> return False) `E.catch` (\(E.ErrorCall e) -> return $ e == s)